The present invention relates generally to signal acquisition probes and more particularly to a signal acquisition probe having a retractable double cushioned probing tip assembly.
Signal acquisition probes acquire electrical signals from a device under test and couple the acquired signal to a measurement instrument, such as an oscilloscope or the like, via an electrical cable. A typical signal acquisition probe acquires voltage signals from a device under test and has a probe head with an electrically conductive hollow tube having a substrate disposed therein. The substrate has passive or active circuitry for conditioning the acquired signal prior to being coupled to the measurement instrument. One end of the hollow tube has an insulating plug disposed therein with a coaxially disposed probing tip extending out of the plug in both directions. The portion of the probing tip extending into the hollow tube is electrically connected to the substrate. Generally, voltage signal acquisition probes are used in hand-held probing of a device under test or mounted to a probing arm that is positioned on the device under test. Excessive force applied to the measurement probe can break the probing tip requiring replacement. Generally, this requires the measurement probe to be sent to a service center where experienced technicians take the probe apart and replace the broken tip. This results in the loss of use of the probe during the repair period and the expense of the repair.
U.S. Pat. No. 6,466,000 describes a replaceable probe tip holder and measurement probe head that allows a user to replace a broken probe tip without sending it to a service center. The replaceable probe tip holder has a cap and attachment arms extending away from the back end of the cap that are positionable on the outside of a probe head housing. The cap has a series of cavities from therein and a bore extending from the innermost cavity to the front end of the cap. A resilient compression member is positioned in the innermost cavity and a probing tip is passed through the resilient compression member and disposed in the bore with the probing point extending outward from the cap. The other end of the probing tip is flared out to form a head that sandwiches the resilient compression member between cap and the probing tip head. Adjacent to the innermost cavity is a second cavity that receives a portion of a substrate that is disposed in a probe head housing. The end face of the substrate has at least a first electrical contact that mates with the head of the probing tip. A third cavity receives a portion of the probe head housing. It should be noted that the probing tip is not securely mounted in the cap of the replaceable probing tip holder and that the probing tip is only securely mounted in the replaceable probe tip holder when the holder is positioned on the probe head.
As the bandwidth of measurement increases, there is a corresponding need for measurement probes having greater bandwidths. A major difficulty in designing very wide bandwidth measurement probes having bandwidths of 5 GHz and greater is the effects of capacitance and inductance of the probing tip or tips. One solution to this problem is to separate the probing tips from the active circuitry in the probing head of the measurement probe. U.S. Pat. No. 6,704,670 describes a wideband active probing system where the probing tip or tips of the probe are separable from a probe amplifier unit. One or more probe cables are connected to a probe tip unit which are connected to the probe amplifier unit for conveying signals received by a probe unit. Various types of probe tip units may be connected to the probe amplifier unit. One type of probe tip unit is a browser probe tip unit which may either be a single ended probe tip unit head or a differential probe tip unit.
The browser probe tip unit includes at least a first typically cylindrical probe barrel. The probe barrel is constructed of an electrically conductive material and extends partially outside of a probe tip unit housing. A probe barrel nose cone is attached to the exposed probe barrel. The probe nose cone is generally conical in form and made of an insulating material. The longitudinal axis of the probe barrel nose cone extends from the probe barrel at an offset angle from the longitudinal axis of the probe barrel. A typically cylindrical shaped probe tip extends partially out of the end of the probe barrel nose cone and is make of an electrically conductive material. A probe cable having an outer shielding conductor and a central signal conductor is connected to the probe barrel and the probe tip with the outer shielding conductor being connected to the probe barrel and the signal conductor being connected to the probe tip. An elastic compressible element engages the probe barrel and the probe tip unit housing allowing movement of the probe barrel into and out of the probe tip unit housing. For a single ended measurement probe, a retractable ground tip is attached to the probe barrel. For a differential measurement probe, two probe barrel and probe nose cone assemblies are positioned side by side in a probe tip unit housing. Individual elastic compressible elements are provided for each assembly. Individual coaxial cables are attached to each assembly.
FIG. 1 shows the forces applied to the probe barrel and probe nose cone assembly or assemblies during use, where “F” is the force applied to the probe nose cone, ΔX is the displacement of the elastic compressible element from its equilibrium position, and K1 is the spring constant. Assuming that the elastic compressible element or elements are pre-loaded, there in an initial force on the assembly or assemblies as represented by the force F1. Downward force on the probe unit housing causes the assembly or assemblies to retract into the probe unit housing. The downward force on the probe unit housing exerts an increasing force K1 on the assembly or assemblies following Hook's Law of F=K1 ΔX where K1 is the spring constant. When the elastic compressible element or elements are completely compressed or the assembly or assemblies engage a fixed stop, continued downward pressure on the probe unit housing transfers forces to the assembly or assemblies as represented by the vertical force line.
U.S. Pat. No. 6,734,689 describes a measurement probe providing signal control for an EOS/ESD protection module. The measurement probe has a spring loaded coaxial probe assembly and a pressure sensor that work in combination to provide an activation signal to the control module. The spring loaded coaxial cable assembly and pressure sensor are disposed in a probe housing. The spring loaded coaxial probe assembly has a semi-rigid coaxial cable with one end forming a probing tip and the other end having a threaded connector. A flexible coaxial is connected to the threaded connector and to the control module. The pressure sensor has a first electrically conductive contact fixed to the coaxial probe assembly and a second electrically conductive contact fixed in the probe housing. A compression spring is positioned over the semi-rigid coaxial cable with one end secured to the semi-rigid coaxial cable and the other end engaging the probe housing. The compression spring is pre-loaded to apply an initial force F1 to the spring loaded coaxial probe assembly as shown graphically in FIG. 1. As downward pressure is applied to probe housing, the coaxial probe assembly retracts into the probe body. The compression spring exerts increasing pressure on the coaxial probe assembly following Hook's Law of F=K1 ΔX where K1 is the compression spring constant. Continued downward pressure applied to the probe housing results in pressure sensor contacts making contact. Since the pressure sensor contacts are fixed to the semi-rigid coaxial cable and the probe housing, any continued downward pressure on the probe housing transfers the forces to the pressure sensor and the coaxial probe assembly as represented in FIG. 1 by the vertical force line. The excess forces on the pressure sensor and the coaxial probe assembly may result in damage to the pressure sensor or the coaxial probe assembly.
A signal acquisition probe for TDR applications is the CP400-04, manufactured by Candox System of Japan. This probe has a metal housing in which an insulated signal conductor is disposed. The metal housing has a threaded connector at one end for connecting a signal cable. The other end of the housing has apertures for receiving spring action pogo pins. One pogo pin is coupled to the insulated signal conductor and the other pogo pins are connected to the metal housing. Assuming that the retractable portion of the pogo pin is pre-loaded, the force on the retractable portion of the pogo pins is similar to that of FIG. 1. The pre-loaded retractable portion has an initial force F1. Downward pressure on the metal housing creates an increasing force on the retractable portion of the pogo pin as represented by line K1. Once all the compression force is taken up in the pogo pin or the pogo pin housing, any further force exerted on the pogo pins is applied to the retractable member as represented by the vertical force line. This may result in damage to the pogo pins.
What is needed is a signal acquisition probe providing a double cushioned retractable probing tip assembly. The signal acquisition probe should provide a safety zone for minimizing probe damage due to excess force being applied to the probing tip assembly. Further, the signal acquisition probe should provide an indication to a user that adequate pressure has been applied to the probing tip assembly so as to prevent damage to the assembly. Additionally, first and second double cushioning retractable probing tips assemblies may be combined in a differential signal acquisition probe.